The present invention relates to a method for treating pulp. In particular, the invention relates to a combined peroxide and acidic treatment of chemical pulp, i.e., both sulphite and kraft (sulphate) pulp, in two separate steps substantially without intermediate washing of any type (including no thickening and no pressing) between the steps, and to a method for effecting such a treatment in a novel and efficient manner.
Peroxide bleaching is conventionally effected at relatively-high temperatures, and requires long bleaching times, e.g., 1 to 7 hours. Normally, the treatment takes 2 to 4 hours since the brightness of the pulp increases slowly during conventional alkaline peroxide bleaching processes, as is shown in FIG. 1. The bleaching process is conducted using peroxide stages. Usually, the peroxide stages needed for bleaching are alkaline. Acidic peroxide stages (having a pH under 5, and preferably under 3) behave in a different manner than alkaline stages. An acidic stage is fast, and usually requires only about 15 minutes for delignification. According to prior art, an acidic peroxide stage is fast and an alkaline peroxide stage is slow (usually taking more than an hour). Surprisingly, it has been found that alkaline peroxide stages having a pH of 9 to 12, preferably 10 to 11 function so that delignification takes place rapidly and bleaching slowly. A peroxide stage effected in neutral conditions (having a pH of 5 to 9) is also fast.
Recent delignification and bleaching trials have now surprisingly revealed that during an alkaline peroxide treatment, the kappa number reduces quicklyxe2x80x94in a matter of a few minutes. An alkaline treatment time of less than 10 minutes, even less than 5 minutes, has been found to be sufficient for reducing the kappa number. FIG. 2 indicates the reduction of the kappa number during time in a peroxide treatment. When a peroxide treatment is employed to reduce the kappa number, and an increase in brightness is not significant or required, then a quick peroxide stage should be sufficient to reduce the kappa number.
Throughout the specification and claims, the following symbols are used for various treatment stages:
xe2x80x9cQxe2x80x9d=heavy metal removal using chelating agents, such as EDTA or DTPA;
xe2x80x9cAxe2x80x9d=heavy metal removal by acid treatment;
xe2x80x9cxe2x88x92xe2x80x9d=a conventional washing or pressing;
xe2x80x9c=xe2x80x9dxe2x80x94a fractionating wash (e.g., an efficient wash using a DRUM DISPLACER(copyright) washer (A. Ahistrom Corporation));
xe2x80x9cOxe2x80x9dxe2x80x94oxygen treatment;
xe2x80x9cOpxe2x80x9d=oxygen treatment with peroxide addition (e.g., 1-10 Kg H2O2/ton pulp);
xe2x80x9cZxe2x80x9d=ozone bleaching;
xe2x80x9cPxe2x80x9d=alkaline peroxide bleaching;
xe2x80x9cPkxe2x80x9d=fast alkaline peroxide bleaching;
xe2x80x9cPoxe2x80x9d=peroxide bleaching with oxygen addition (e.g., 1-10 Kg O2/ton pulp);
xe2x80x9cDxe2x80x9d=chlorine dioxide treatment;
xe2x80x9cIxe2x80x9d=either or both of the steps on either side thereof; and
where two or more steps are listed right next to each other, there is substantially no intervening washing of any type (e.g. no displacement washing, no thickening).
According to one aspect of the present invention a method of treating chemical pulp with at least hydrogen peroxide or the equivalent, is provided, wherein the kappa number of the chemical pulp has been brought to a level below 20 by cooking, or cooking and delignification, or the like. The method comprises the steps of: (a) treating the pulp in a fast alkaline, neutral, or acidic peroxide step (Pk) to reduce the kappa number by 1-10 kappa units and in which step the reaction time is less than 10 minutes, (b) transferring the pulp, substantially without intermediate washing of any type, to a bleaching tower, and (c) bleaching the pulp further in an acidic bleaching step in the bleaching tower. During step (b) acidic bleaching chemical may be mixed with the pulp, and step (c) may be practiced using chlorine dioxide or ozone as the acidic bleaching chemical.
The method may also comprise the further step (d) of treating the pulp with chlorine dioxide after the practice of steps (a) through (c). Further in step (c) the pulp may be treated in the Pk and ozone stages to decrease consumption of chlorine dioxide in step (d).
Steps (a) through (c) may be practiced in accordance with the following partial treatment sequence: A-PkDQ-Po, in which Pk is either Pk alone, PkO, or PkOp and in which the A stage is replaceable with a Q or D stage. The sequence may be preceded by an oxygen delignification stage. Ideal conditions in step (a), especially for kraft pulp, are alkaline pH, 78-110xc2x0 C., about 8-20% pulp consistency, and a time of less than about 5 minutes, and a kappa number reduction of 3-7 units.
Alternatively, steps (a) through (c) may be practiced utilizing the following partial treatment sequence: A-PkD-Po, and A-PkD, in which Pk is either Pk alone, PkO, or PkOp and in which the A stage is replaceable with a Q or D stage. There may also be the further step, before step (a), of oxygen delignifying the pulp in at least one oxygen delignification stage. At least one of the D stages may be supplied with Mg and/or Ca to adjust the metal profile to a desired value.
According to another aspect of the present invention steps (a) through (c) may be practiced utilizing the following partial treatment sequence: A-PkD-E-D; A-PkZDN-Dxe2x80x2 and A-PkZ-D; which Pk is either Pk treatment alone, or PkO, or PkOp and in which the A stage is replaceable with a Q stage, i.e., chelation or with a D stage; or steps (a) through (c) may be practiced with the following other partial treatment sequences:
O-A-PkD-E-D; O-A-PkZDN-D; and O-A-PkZ-D; in which Pk is, besides Pk alone, also treatment PkO, or PkOp, and wherein the A stage is replaceable with a Q stage, i.e., chelation or with a D stage;
A-PkAZQ-Po; and A-PkZQ-Po; wherein Pk is, besides Pk alone, also treatment PkO, or PkOp and in which the A stage is replaceable with a Q stage, i.e., chelation, or with a D stage;
A-PKAZ-Po; and A-PkZ-Po; wherein Pk is, besides Pk alone, also treatment PkO, or PkOp, and in which the A stage is replaceable with a Q stage, i.e., chelation, or with a D stage; or
O-A-Pk-ZQ-Po; and O-A-PkZQ-Po; wherein Pk is, besides Pk alone, also treatment PkO, or PkOp and in which the A stage is replaceable with a Q stage, i.e., chelation, or with a D stage.
In any of the above sequences the A stage of the treatment sequence may be a DQ or a D stage which is supplied with chemicals adjusting the metal profile, selected from the group consisting essentially of Mg, Ca, DTPA, EDTA, NTA, and HEDTA.
The method steps of the invention may be practiced with kraft (sulphate) pulp, or with sulphite pulp. When practiced with sulphite pulp metal removal prior to the Pk stage is not necessary, or utilized.
According to another aspect of the present invention there is provided: A first small (i.e. a maximum volume of 90 m3 for 1500 tons per day pulp production, preferably about 45 m3 or even less) reactor in which pulp having a kappa number below 20 is treated in a fast alkaline, neutral, or acidic peroxide step (Pk) to reduce the kappa number by one to ten kappa units, and in which the reaction time is less than ten minutes. A second reactor comprising a bleaching tower. A conduit connecting the first reactor to the second reactor, the conduit including substantially no intermediate washing device of any type. And apparatus which effects acidic bleaching in the second reactor. A mixer for mixing acidic bleaching chemical with the pulp may be provided in the conduit between the reactors (e.g. bleaching towers), as part of the apparatus which effects acidic bleaching (along with conventional feeds, withdrawals, and outlets of and for the second reactor).
It is the primary object of the present invention to provide effective treatment of chemical pulp including one which is very favorable in terms of overall economy of treatment. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.